I. Field of the Invention
The present invention relates to a high density, controlled impedance connector and, more particularly, to a high density connector which may be utilized to mate a plurality of modules (daughter boards) to a backplane (mother board) wherein each electrical connection has a controlled impedance and a minimum amount of crosstalk.
II. Description of the Prior Art
In the prior art, there has been a considerable amount of discussion of the utilization of a flat cable system which migh include a flat or round wire for handling high speed signals such as high speed digital pulses. The advantage of the flat cable is that one or two sides of the cable may be provided with a conductive layer which, in turn, is connected to ground. If a single conductive layer is used on one side, a microstrip is formed. When conductive layers are used on both sides, a stripline is formed. For an article discussing the mathematics and properties of such flat cables, see: Bossi, Dennis F., Testing Electrical And Transmission Properties In Flat Cable, presented at the 19th International Wire & Cable Symposium, Atlantic City, N.J., December, 1970.
A flat cable formed with a plurality of flat conductors and surrounded on its upper and lower surface by a ground plane, thus forming a stripline, may be found in U.S. Pat. No. 3,612,744, issued Oct. 12, 1971 and invented by P. J. Thomas. A second flat cable in the form of a microstrip is described in U.S. Pat. No. 4,441,088, which issued Apr. 3, 1984 by C. J. Anderson. The Anderson patent discusses the reduction of crosstalk by adjusting the amount of dielectric material between the flat conductor and the ground plane in proportion to the amount of dielectric material placed over the flat conductor.
The advantage of utilizing a flat cable becomes apparent after consideration of the discussions within the references cited above. That is, the dimensions of the cable may be altered to select or control the impedance and to reduce the amount of crosstalk. This concept was incorporated into an early connector wherein a dielectric sheet of resilient material was surrounded on one side by a ground plane and on the other by conductive strips. The distance between the conductive strips and the resilient ground plane was said to achieve impedance matching characteristics. See U.S. Pat. No. 3,401,369, issued Sept. 10, 1968 by P. H. Palmateer, et al.
A later connector for shielding electrical contacts therein to permit a high frequency signal to pass there through utilizing a stripline configuration is shown in an IBM Technical Disclosure Bulletin, Volume 10, No. 3, August 1967, pp. 203-4. This connector does not contemplate a high density connector as in the present invention.
It is also known in the prior art to use a connector having a plurality of contacts mounted directly into a mother board. These contacts mate with conductive elements upon the mother board and include spring fingers that wipe conductive elements on a daughter board to make electrical connection between the daughter board and the mother board. In one such connector, shown in U.S. Pat. No. 3,651,432, issued Mar. 21, 1972, by H. E. Henschen, et al., impedance matching of a microstrip circuit is accomplished by connecting a middle contact to a signal carrying element on the mother board while connecting contacts on either side thereof to a ground plane on the opposite side of the mother board. In this configuration, the signal carrying contact is surrounded by a ground connection to control and match impedance. However, this configuration is extremely bulky and does not lend itself to a high density connector system.
Two additional connector systems utilizing round wires which are bent at ninety degrees to form contacts that are inserted into a mother board are shown in U.S. Pat. No. 4,070,084, issued Jan. 24, 1978, by R. V. Hutchison, and U.S. Pat. No. 4,232,929, issued Nov. 11, 1980, by F. Zobawa. The first patent discusses a means for controlling impedance using a microstrip arrangement by imbedding a conductive element within a dielectric substrate. An alternative arrangement shows a flexible dielectric material with a ground plane on one side and conductive elements on the other. The second patent discusses reduction of crosstalk by an intermediate ground plane located between the contacts. Each of these arrangements suffer from bulk and inability to produce a high density connector.